The present invention relates to an improvement in controls for single phase motors and, more particularly, to circuitry for reducing the flux in a single phase motor during light load operation to thereby reduce losses and increase efficiency of the motor.
Many electrical motors perform at less than their rated loads during a majority of the time they are energized. Efficiency of an electric motor declines with load and therefore an increasing proportion of the electrical energy delivered to a motor under reduced load is wasted. Since single phase motors comprise a substantial part of the total electric motor population, it is therefore desirable to reduce the energy lost during light load operation of these motors.
It has been recognized before that under reduced load conditions the magnetic flux in a single phase motor can be reduced because the torque requirements necessary to sustain the load are also reduced. A significant portion of the total losses in a single phase motor is due to losses occurring in the motor core. By reducing the flux at light loads, the losses in the motor core are also reduced thereby improving the light load efficiency of the motor. Reduction in flux also provides improvement in motor efficiency by reducing the magnetizing component of the current and thereby also reducing the i.sup.2 R losses of the motor. As a result, the efficiency of a motor can be increased by controlling the flux in a single phase motor during light load conditions.
One well known and convenient means for controlling the flux of a motor is accomplished by inserting a triac, or equivalent switching device, in series with the motor. By reducing the "on time" of the switching device as the load decreases, the effective value of the voltage applied to the motor and therefore the flux may be reduced thereby increasing motor efficiency. In order that "on time" of the switching device be controlled as a function of the load, it was considered necessary that the load be directly measured by a separate sensing device. The use of such a sensing device, however, adds more expense and complexity than can be justified for most applications where single phase motors are used, even when compared to the energy saved. As an example, rotor speed is a good indicator of the load on a single phase motor. A control circuit would need simply to maintain a constant RPM as the load varies. For this type of control, a tachometer is required in order to sense the speed.
Other methods of increasing efficiency of single phase motors include power factor control, wherein a constant power factor is attempted to be maintained as the load varies. If the power factor can be maintained, the motor would be operating near its maximum efficiency. The power factor, however, is not monotonic as a function of voltage. Further, for rapid changes in load, it is possible that the circuitry cannot respond fast enough to prevent the power factor from exceeding the maximum value and driving the motor voltage in the wrong direction.
While it was recognized at this time that it would be far more desirable to control the "on time" of the switching device by a parameter which is an indicator of load but does not require the use of separate load sensing devices, no such technique was known.